/Engine/Source/Runtime/Experimental/GeometryCollectionEngine/Public/GeometryCollection/GeometryCollectionISMPoolComponent.h
Create an Mesh group which represent an arbitrary set of mesh with their instance no resources are created until the meshes are added for this group return a mesh group Id used to add and update instances
[FMeshGroupId](API\Runtime\GeometryCollectionEngine\GeometryCollection\UGeometryCollect-_3\FMeshGroupId) CreateMeshGroup()